High-gloss surface by means of hot-coating

ABSTRACT

The present invention relates to a process for producing high-gloss surfaces on at least one portion of a substrate area, where the steps comprise (a) applying a layer made of a melt to at least one portion of the substrate area; (b) polishing of the applied layer of melt; (c) applying at least one lacquer layer to the polished layer of melt by means of a curtain-coating process; and (d) hardening the layer structure applied. The invention further relates to articles obtainable by this type of process.

The present invention relates to a process for producing high-glosssurfaces and to articles with surfaces of this type.

There has recently been an increase in demand for high-gloss surfacesfor element surfaces in the furniture- and wood-processing industry, andalso in the production of floors.

This surface gloss is an optical perception which arises when a surfaceis viewed, an object then being perceived as glossy if most of the lightincident on the surface is reflected directionally and only a smallportion is reflected diffusely. High-quality high-gloss surfaces areperceived as having gloss similar to that of a glass surface. The glosslevel can by way of example be determined by using appropriate glosslevel measurement equipment to determine reflectance for a light beamincident at a defined angle. The term high gloss usually means a glosslevel of from 80 to 100 at a test angle of 60°.

It has hitherto been disclosed that appropriate high-gloss surfaces canbe produced by laminating high-gloss foils in the form of specificplastics sheets or plastics foils onto substrate sheets. Substratesheets of a wide variety of forms and comprising a wide variety ofmaterials can be coated by this method. However, the variety ofdecorative effects available on these plastics sheets and plastics foilsis restricted, and the associated costs are high.

In a process that is likewise known, a finishing process is undertakenbefore the production of a substrate sheet is completed. Sheets oftimber-based material, for example MDF or HDF, are coated with paper,preferably what are known as melamine papers, these being applied to thesheet with use of melamine resin and sealed, giving a directly coatedsubstrate sheet. In order to achieve a desired high-gloss surface,transparent plastics foils are laminated onto substrate sheets preparedin this way. A disadvantage here is that the lamination material appliedmust be applied very homogeneously, with uniform layer thickness. Theplastics foils that can be used moreover have a significant thickness inorder to compensate possible unevenness of the substrate sheets orunevenness in application of the lamination material, and this increasescosts.

The use of lacquers to produce a high-gloss surface on appropriatelyprepared, directly coated substrate sheets has hitherto been possibleonly at high cost, since lacquers exhibit poor adhesion properties onmelamine layers. Although lacquers exhibit advantageous properties, forexample rapid drying and high resistance to moisture and to cleaningcompositions, they have therefore seldom been used hitherto in theproduction of high-gloss surfaces.

The low adhesion of lacquers often derives from their chemical and/orphysical nature and from the nature of the surface to be coatedtherewith, and a known method here achieves increased adhesion throughpretreatment of the surface. Adhesion promoter layers are used here;they improve the adhesion properties of the elements that are to bejoined, and they can moreover compensate unevenness on the surfaces.

DE 10 2009 002 048 A1 discloses the use of a reactive polyurethane-basedmelt as thin adhesion promoter layer in conjunction with a lacquer layerthat is then to be applied. However, there is no disclosure of the useof an adhesion promoter layer in conjunction with the production of ahigh-gloss surface.

Lacquer systems based on UV-curing lacquers are in general used in theproduction of furniture and of floors, and are applied to theappropriate parts by using rolls, or less frequently by using spraymethods. The hardening procedure that follows uses UV light/UV lamps.The cost of machinery for lacquering is very high, as also is the spacerequired. This is partially due to multilayer lacquer application—fromthree to four applications being usual in practice—since for reasonsrelating to viscosity and to thorough UV curing each lacquer applicationcan only apply a very thin layer of about 10 to 20 μm. Furthermore, anumber of intermediate lacquer polishing operations are mostly needed toachieve a surface having the smoothness required for high-glossapplications.

In particular, it is possible to achieve highly abrasion-resistanthigh-gloss surfaces, for example with abrasion resistance in theabrasion class>AC5, by adding abrasive materials in the lacquer systemsused, an example being aluminum oxide particles or ceramic particles.When corundum-containing lacquer systems of this type are used, it isdifficult to carry out intermediate polishing operations, and is nottherefore possible to achieve surfaces with appropriately high gloss.

It is therefore expensive to combine high abrasion resistance, requiredby way of example for the floor sector or for a worktop, and the desirefor a highly polished, high-gloss surface.

It is therefore an object of the present invention to provide a moreefficient process for producing high-gloss surfaces, avoiding theabovementioned disadvantages.

The object is achieved via a process for producing high-gloss surfaceson at least one portion of a substrate area, where the steps comprise:

-   a) applying a layer made of a melt to at least one portion of the    substrate area;-   b) polishing of the applied layer of melt;-   c) applying at least one lacquer layer to the polished layer of melt    by means of a curtain-coating process;-   d) hardening the layer structure applied.

The object is further achieved via an article with a high-gloss surfaceon at least one portion of a substrate area, obtainable by a process ofthe invention.

Surprisingly, it has been found that the use of a melt, in particular areactive polyurethane-based melt on a substrate area, in particular amelamine-paper-coated timber particleboard or melamine-paper-coatedMDF/HDF, provides a homogeneous and smooth surface which moreoverimproves the conditions for a lacquer layer to be applied in asubsequent step and thus permits production of a high-gloss surface ofhigh quality.

In particular, a high-gloss surface of this type can be achieved evenwhen the melamine surface has a certain texture and roughness depth, asis by way of example usually the case with commercially available sheetmaterials provided with a standard surface, these often having astructure, e.g. a mini-pearl structure. Accordingly, amelamine-laminated sheet material does not have to be prepared in themanner that is necessary with some lacquering processes.

The melt can be based on polyolefins, ethylvinyl acetates (EVAs),polyamides, or polyurethanes. It is preferable that the melt involves areactive melt, in particular based on a polyurethane.

The preferred reactive polyurethane-based melt preferably involves acommercially available reactive polyurethane-based melt which preferablyreacts and hardens by virtue of the moisture present in the atmosphere.It is preferable that said reactive melt is one that is now used for thepurposes of “hotcoating” technology for the finishing of surfaces, beingfree from water and from solvent. A feature of these reactivepolyurethane-based melts is not only simple application but alsoresistance to water vapor, moisture, and chemicals, and to mechanicalstress. These reactive melts have a high level of adhesion-promoterproperties, and only thin layers are therefore needed. In this contextit is in particular surprising that an applied layer of reactive meltcan harden although, for example, a lacquer layer separates this fromthe atmosphere and the moisture present therein which causescrosslinking and thus hardening.

Another surprising factor is that melts comprising highlyabrasion-resistant, abrasive materials can also be used here. Melts ofthis type can be processed similarly, giving high layer thicknesses, forexample in a range from 60 to 100 μm. Surprisingly, a high-gloss surfaceis achieved without a requirement for intermediate polishing steps. Inparticular, it is possible to produce highly abrasion-resistanthigh-gloss surfaces which by way of example have abrasion resistance inthe abrasion class>AC5 in accordance with DIN EN 13329. Suitableabrasive materials are by way of example aluminum oxide particles orceramic particles. A corundum-containing melt is preferred.

Suitable substrate areas are element surfaces, for example for laminatefloors, for floorboards, or for parquet floors, for panels or veneers,or for furniture or items of furniture. The substrate areas can becomposed of wood or of wood-like material, for example of a veneer orfiberboard, where these can previously be covered with paper, withplastic, or with other materials.

The melt can be applied by a roll method or spray method, or by means ofa nozzle or slot die, to the area requiring coating, in particular to atimber particleboard provided with a melamine layer or to MDF/HDFprovided with a melamine layer. It is preferable that the melt isapplied by means of a roll apparatus to an appropriate surface. For thispurpose, the substrate areas, e.g. in the form of substrate sheets, aretransported by means of a transport apparatus in a particular transportdirection and with a transport velocity past the roll apparatus, wheremelt is applied by means of at least one metering roll and oneapplicator roll, which rotates with a velocity in the range from 5 to 60m/min, preferably from 10 to 25 m/min. The layer of melt applied is thenby way of example polished by means of a downstream polishing roll whichrotates in opposition to the applicator roll with a velocity in therange from 1 to 30 m/min, preferably from 1 to 6 m/min. It has provenadvantageous to carry out application with exclusion of air and,respectively, isolation from moisture present in air.

The thickness of the layer of melt is preferably in the range from 5 to200 μm, with preference from 10 to 60 μm.

The melt is usually applied at a temperature of at least 100° C., forexample from 100° C. to 150° C., preferably from 120° C. to 150° C. Byway of example, a correspondingly temperature-controlled applicator rollis used, and the temperature of the polishing roll here, which canlikewise be a heatable roll, can differ from that of the applicatorroll.

A layer of a high-gloss lacquer is then applied, preferably beforecomplete hardening of the layer of melt, to the layer of melt that hasbeen applied and polished, and a significant time saving is thusachieved. The layer of melt applied provides a surface on which thehigh-gloss lacquer to be applied exhibits good flow properties andhomogeneous distribution.

In the step that now follows, the high-gloss lacquer is applied to thesubstrate area thus prepared, and it is preferable in the invention hereto use what is known as a wet-on-wet method. It is preferable to use apourable UV-curing high-gloss lacquer, an electron-beam-curing lacquer,a ceramic lacquer, and/or an aqueous lacquer, applied by means of theknown curtain-coating process. The substrate area prepared by means ofpreceding steps is passed here through a lacquer curtain composed ofliquid lacquer, whereupon lacquer is applied to give a coating in therange from 100 to 160 g per square meter.

After the smoothing of the layer of melt (step b)) and before theapplication of the high-gloss lacquer by the curtain-coating process(step c)), at least one first lacquer layer can optionally be appliedbefore at least one further layer follows in step (c). This can involveany desired lacquer which advantageously hardens rapidly, to permitquick further processing. Suitable lacquers used can by way of examplebe clear, white, or colored lacquer, for example in the form ofnitrocellulose lacquers, electron-beam-curing lacquer, ceramic lacquer,aqueous lacquers, or 2C PU lacquers. However, it is preferable to useUV-curing lacquers. This type of lacquer-application process can apply alayer of thickness about 10 to 20 μm and it is therefore possible toachieve thorough UV curing within a reasonable time. The optionallacquering step can be achieved by means of conventional processes, forexample application by a roll method, or by the spray process, or inkjet. This optional step provides a very smooth substrate area on which afurther lacquer layer can be applied in the following step, in order byway of example to achieve a high-gloss surface of the highest quality.

After application of the high-gloss lacquer, the layers applied arehardened, and it is preferable here to observe a waiting time inparticular of some minutes, for example at least 1 minute, in order thatthe at least one lacquer layer becomes homogeneously distributed on thesurface.

The at least one lacquer layer applied, preferably a UV-curing layer, ishardened via irradiation with the aid of commonly used apparatuses, forexample UV lamps. In order to produce a high-gloss surface of highquality it is important that during the hardening procedure the surfacecovered remains as far as possible free from undesired particles. Tothis end, a superatmospheric pressure can be generated in theirradiation apparatus, preferably provided in the form of a tunnel, sothat ingress of foreign particles, e.g. dust particles, is avoided.

What is claimed is:
 1. A process for producing high-gloss surfaces on atleast one portion of a substrate area, wherein the substrate area iscomposed of wood or wood-like material and the substrate area is coveredwith melamine paper, where the steps comprise: a) applying a layer madeof a reactive melt to at least one portion of the substrate area coveredwith melamine paper; b) polishing the applied layer of melt; c) applyingat least one lacquer layer to the polished layer of melt by means of acurtain-coating process; and d) hardening the layer structure applied.2. The process as claimed in claim 1, wherein the reactive melt is basedon a polyurethane.
 3. The process as claimed in claim 1, wherein themelt is applied in step a) by means of a roll apparatus comprising atleast one metering roll and an applicator roll rotating in a directionof rotation.
 4. The process as claimed in claim 3, wherein theapplicator roll rotates with a velocity in the range from 5 to 60 m/min.5. The process as claimed in claim 4, wherein the applicator rollrotates with a velocity in the range from 10 to 25 m/min.
 6. The processas claimed in claim 3, wherein the melt applied in step a) is polishedby a polishing roll rotating in opposition to the direction of rotationof the applicator roll.
 7. The process as claimed in claim 6, whereinthe polishing roll rotates with a velocity in the range from 1 to 30m/min.
 8. The process as claimed in claim 7, wherein the polishing rollrotates with a velocity in the range from 1 to 6 m/min.
 9. The processas claimed in claim 1, wherein the melt applied in step a) comprises anabrasive material.
 10. The process as claimed in claim 1, wherein thelacquer layer to be applied in step c) comprises a pourable UV-curinghigh-gloss lacquer.
 11. The process as claimed in claim 1, wherein priorto step c) at least one first lacquer layer comprising a UV-curinglacquer is applied with the aid of a roll, by way of spray equipment, orby means of ink jet.
 12. The process as claimed in claim 1, wherein stepc) uses a wet-on-wet method.
 13. The process as claimed in claim 1,wherein prior to step d) a waiting time is observed for hardening. 14.The process as claimed in claim 1, wherein the substrate area coveredwith melamine paper is an element surface of a laminate element, of aparquet element, or of a floorboard element, of a panel, or of a veneer,furniture, or an item of furniture.